U.S. Pat. No. 4,821,066 issued Apr. 11, 1989 to James C. Foote, Jr., Frank Castrignano and Robert D. Huot shows a color printer having an electrophotographic endless belt entrained about a series of rollers. The endless belt has a series of perforations along one edge. A printhead roller sprocket and a transfer sprocket are provided for engaging an edge of each perforation to provide registration between the printhead and the transfer roller for the production of high quality color reproduction. The transfer sprocket also provides the drive for the belt by engagement with the perforations of the belt. In this patent errors in image formation, due to perforation formation error, are corrected by having the same perforation control both transfer and image formation.
The above registration and drive system, however, is highly reliant upon tolerances and continued integrity of the belt perforations for quality imaging, as indicated by the following:
1. The teeth of the printhead roller sprocket are to be constantly engaging an edge of the perforations to assure that the angular position of the printhead roller sprocket is representative of exact belt position;
2. The pitch of the sprocket teeth, as determined by the pitch of the perforations, must be such that only the proper tooth of the transfer sprocket and the printhead roller sprocket control location of the transfer surface and exposure, respectively; and
3. Very tight tolerances, + or -0.015 mm, on the length of each perforation must be maintained if the teeth of the transfer sprocket engage one edge of the perforation and the teeth of the printhead roller sprocket engage another edge of the perforation.
Errors, of even a small magnitude in the above, which may be the result of damage to the perforation caused by the driving sprocket, may result in loss of resolution, poor image color alignment and may also result in a shift in the hue of colors that result from the combination of two different toners.
Even though the above disclosed system works very well, there is a chance of the perforations wearing due to the constant engagement of the sprocket teeth with the performations as the sprocket drives the belt. This could, without belt replacement, affect registration of the system.